1. Field of the Invention
The present invention is directed to fuel injection systems, and more particularly to piezoelectric injection systems that function independently of injector pressure and operating temperature.
2. Description of Related Art
In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to inject fuel pulses into the engine combustion chamber. A commonly used injector is a closed-nozzle injector which includes a nozzle assembly having a spring-biased nozzle valve element positioned adjacent the nozzle orifice for allowing fuel to be injected into the cylinder. The nozzle valve element also functions to provide a deliberate, abrupt end to fuel injection, thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The nozzle valve is positioned in a nozzle cavity and biased by a nozzle spring so that when an actuated force exceeds the biasing force of the nozzle spring, the nozzle valve element moves to allow fuel to pass through the nozzle orifices, thus marking the beginning of the injection event.
Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. As such, one aspect of fuel supply systems that has been the focus of designers is the use of piezoelectric actuators in fuel injectors.
In general, piezoelectric actuators have long been recognized as highly desirable for use in systems requiring extremely fast mechanical operation in response to an electrical control signal. For this reason, piezoelectric actuators have received considerable attention by designers of fuel supply systems for internal combustion engines. Such designers are continually searching for ways to obtain faster, more precise, reliable, and predictable control over the timing and quantity of successive fuel injections into the combustion chambers of internal combustion engines to help meet the economically and governmentally mandated demands for increasing fuel economy and reduced air pollution. If such goals are to be attained, fuel control valves must be designed to provide extremely fast and reliable response times.
As discussed hereinbelow, conventional fuel injectors with piezoelectric actuators, however, suffer from notable disadvantages. For instance, the inherent design limitations of conventional piezoelectric fuel injectors make it more difficult to achieve certain performance characteristics, such as increased injection pressures. Moreover, the performance of conventional piezoelectric actuators is affected by environmental and operational factors, such as temperature.
Piezoelectric devices are capable of extremely fast and reliable valve response times. As a result, they offer greater control over fuel delivery, because they can be used to inject required amounts of fuel in a short time frame. The time frame for injecting fuel can be shortened by injecting the fuel at higher injection pressures. For instance, manufacturers have implemented extra high pressure injection systems, also known as XPI, where the pressures can reach 2400 bar. Such high injection pressures create smaller fuel droplets and higher injection velocity to promote more complete burning of the fuel, which maximizes power and increases fuel economy. In addition, pollution is minimized because the high thermal efficiencies result in low emissions of hydrocarbons (HC) and carbon monoxide (CO). By injecting required amounts of fuel in a shorter time frame, a high pressure system can accommodate multiple injection events during each combustion cycle. As a result, the engine control software can optimize combustion for particular conditions.
The use of very high injection pressures, however, requires piezoelectric actuators of conventional fuel injectors to operate with correspondingly high force levels. In general, piezoelectric actuators must act against the high pressure fuel in the fuel injector to move the nozzle valve into an open position causing the injection of fuel. For instance, in one type of fuel injector design, a control chamber filled with high pressure fuel is employed to bias the nozzle valve in the closed position against the force of a spring, and the piezoelectric actuator opens a control valve to expose the control chamber to a low pressure drain. When the fuel drains from the control chamber, the pressure in the control chamber drops and is no longer able to keep the nozzle valve in the closed position. In order to open the control valve, the piezoelectric actuator must act against the high pressure in the control chamber. Thus, piezoelectric actuators in such fuel injectors must provide large forces due to the high pressure which exists in the fuel injector.
Accordingly, the design of conventional piezoelectric actuators is dependent on the injector pressures. High pressure injection fuel injectors are required to use larger piezoelectric actuators to supply the necessary forces. Moreover, more power is required to operate conventional piezoelectric actuators with high injection pressures.
As mentioned previously, the performance of conventional piezoelectric actuators is also affected by environmental and operational factors, such as temperature. When used as a valve actuator, piezoelectric devices are known to provide extremely fast, reliable characteristics when calibrated to and operated at a relatively constant temperature. However, internal combustion engines are required to operate reliably over an extremely broad ambient temperature range. Moreover, fuel injection valves mounted directly on the engine are subjected to an even broader range of temperatures since the operating temperatures of an internal combustion engine may extend well above ambient temperatures and may reach 140° C. or more. Such temperature extremes can produce wide variations in the operating characteristics (e.g. length of stroke and/or reaction time) of a piezoelectric actuator. Conventional piezoelectric injectors have always experienced shifts in fueling due to temperature and the difference in the thermal expansion between the piezo ceramic and the material used to mount the piezo. In particular, the ceramic thermal coefficient of expansion is much lower than that for steel. Because the useable stroke of a piezoelectric actuator is in the 30 to 40 micron range, the thermal effects can exceed the stroke. Such actuator variations can lead to wide variations in timing and quantity of injected fuel when the piezoelectric actuator is used to control fuel injection into an internal combustion engine. Thus, conventional piezoelectric fuel injectors are affected by typical temperature variations in an engine.